Computational Photography

The Flutter Shutter

Patterns encoded in a different dimension—time rather than space—provide a strategy for coping with motion blur. Raskar, Agrawal and Tumblin have built a camera that removes the fuzzy streak created by an object that moves while the shutter is open.

Undoing motion blur would seem to be easier than correcting misfocus because motion blur is essentially one dimensional. You just gather up the pixels along the object's trajectory and separate the stationary background from the elements in motion. Sometimes this method works well, but ambiguities can spoil the results. When an object is greatly elongated by motion blur, the image may offer few clues to the object's true length or shape. Guessing wrong about these properties introduces unsightly artifacts.

A well-known trick for avoiding motion blur is stroboscopic lighting: a brief flash that freezes the action. Firing a rapid series of flashes gives information about the successive positions of a moving object. The trouble is, stroboscopic equipment is not always available or appropriate. Raskar and his colleagues have turned the technique inside out. Instead of flashing the light, they flutter the shutter. The camera's shutter is opened and closed several times in rapid succession, with the total amount of open time calculated to give the correct overall exposure. This technique converts one long smeared image into a sequence of several shorter ones, with boundaries that aid in reconstructing an unblurred version.

A further refinement is to make the flutter pattern nonuniform. Blinking the shutter at a fixed rate would create markers at regular intervals in the image, or in other words at just one spatial frequency. For inferring velocity, the most useful signal is one that maximizes the number of distinct spatial frequencies. As with a coded aperture, this coded exposure adds an identifiable signature to the image data.

Some cameras come equipped with a mechanical stabilizer designed to suppress a particular kind of motion blur—that caused by shaking of the camera itself. The shutter-flutter mechanism could handle this task as well.